I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to frequency planning in a cellular network.
II. Description of the Related Art
Cellular radiotelephone systems enable mobile subscribers to communicate with land-line telephone networks while moving through a geographical area. High density, high capacity cells in typical cellular radiotelephone systems are made up of three directional antennas centrally located in a tri-cell group comprising of three cells each 120.degree. from the others. Each antenna radiates into a 120.degree. sector of the tri-cell group. A number of these tri-cellular groups are combined to form the cellular radiotelephone system.
The cell shapes are determined by both the radiation pattern of the antennas and the local conditions at the cell site. Cells, however, are typically idealized as hexagonal patterns since such a pattern closely approximates the ideal antenna radiation pattern.
Cellular radiotelephone systems use different channel frequencies for each mobile subscriber. The transmission from the mobile to the cell uses one frequency while the transmission from the cell to the mobile user uses another frequency. These two frequencies are not used by other nearby mobiles as this would lead to interference on the channel and a reduction in signal quality. This interference is referred to in the art as co-channel interference.
Another type of interference experienced by mobile subscribers is adjacent channel interference. This interference is due to the energy slipover between adjacent channels.
Both types of interference affect the signal quality, referred to as the carrier to interference ratio (.sup.C /.sub.I). This ratio is the signal strength of the received desired carrier to the signal strength of the received interfering carriers. A number of physical factors can also affect .sup.C /.sub.I in cellular systems including: buildings, geography, antenna radiation patterns, mobile traffic transmitting power, and mobile traffic location within the cell.
Due to the low power of the cell's transmitters, the same frequencies can be reused in other cells, referred to as co-channel cells, in the same geographical area. Greater frequency reuse allows more mobile traffic to use the cellular system. There are, however, constraints on the location of the co-channel cells. Even though the transmitters are typically low power, placing co-channel cells too close may cause interference.
Frequency planning optimizes spectrum usage, enhances channel capacity and reduces interference. A frequency plan also ensures adequate channel isolation to avoid energy slipover between channels, so that adjacent channel interference is reduced. Moreover, an adequate repeat distance is provided to an extent where co-channel interference is acceptable while maintaining a high channel capacity. In order to accomplish these diverse requirements, a compromise is generally made so that the target .sup.C /.sub.I performance is acquired without jeopardizing the system capacity.
FIG. 1 illustrates a typical prior art N=4, 120.degree. tri-cell group. This system is based on dividing the available channels into 12 frequency groups that are distributed evenly among four 120.degree. tri-cell groups (a cluster) comprising of 12 cells. One frequency group is allocated per cell.
The cluster of FIG. 1 is used to construct a cellular network. Each cluster is repeated throughout the network. As a result, there are multiple co-channel sites that are oriented in the same direction. The C/I performance, therefore, is due to those co-channel interferers.
In the above example, the co-channel interferers would normally cause problems for quality communications in this system. In this case, antenna downtilt and beam width, have to be properly engineered for satisfactory performance and operation of this system.
To meet subscriber growth requirements, modifications to existing frequency plans and adoption of different cluster schemes are required. For example, one system may have to be transformed from an N=8 system to an N=4 system.
When the new frequency plans have to be deployed in existing cellular networks, the changes have to be done in phases, taking into account the additional capacity requirements. Additionally, the transition has to take place smoothly and coexist with the existing scheme with minimum disturbance to the cellular network and subscribers. There is a previously unknown need for a method for transitioning from one frequency plan to another frequency plan without disrupting the cellular network.